This paper studies the coupled mass and heat transport as well as the reactions in an indirect internal reforming (IIR) unit of a molten carbonate fuel cell (MCFC). The aims of the work are first to identify the dominating transport processes for a specific design. Because the temperature field is one major issue in MCFCs, the second aim is to predict the spatially distributed temperature field within the unit. In a first step, several variants of a microscale model, describing only a small detail of the IIR unit, are created. The governing equations and the boundary conditions of this model are given. The results of these simulations, especially the temperature and concentration distributions, are discussed. They show that the gas phase is divided into a reactive and a nonreactive zone in the actual design of the IIR and the reforming process is dominated by mass transfer between these zones. In a second step, a macroscale model of the entire IIR unit is presented. It considers a simplified geometry, but it incorporates the two gas zones identified by the microscale model.